1. Field of the Invention
The present invention relates to a protective agent for an image carrier, an image forming apparatus, and a process cartridge.
2. Description of the Related Art
As is generally known, in an electrophotographic image forming system, an electrostatic latent image is formed from electrostatic charges on an image carrier having a photoconductive layer including, for example, a photoconductive substance, and a charged toner is permit to adhere to the electrostatic latent image to form a visible image. The visible image is transferred onto a recording medium, such as paper, and then fixed to the recording medium by heat, pressure, or solvent vapor, obtaining an output image.
The image formation is roughly classified, according to the method for charging the toner to form a visible image, into a two-component development system using frictional charging by stirring and mixing the toner and carrier and a one-component development system in which the toner is charged without using a carrier. The one-component development system is further classified into a magnetic one-component development system and a non-magnetic one-component development system according to the use of magnetic force in keeping the toner on a development roller.
In a copier or a multifunction product having the function of a copier, which is required to have high speed and excellent image reproducibility, the two-component development system has been widely used to meet the demands where the machine should have excellent toner charging stability, excellent starting up properties, or long stability of image quality. On the other hand, in a small-size printer or facsimile which is strongly required to occupy a less space or reduce the cost, the one-component development system has been widely used.
Recently, in any development systems, color output images have widely spread, and there are further increasing demands where the image should be improved in quality or the image quality should be stabilized. For achieving high-quality images, the toner is reduced in an average particle diameter, or the toner particle is rounded and has no sharp edges.
In a general electrophotographic image forming apparatus, regardless of the development system, an image carrier in a drum form or a belt form is uniformly charged while rotating, and a latent image is formed on the image carrier using, for example, a laser beam. The latent image is developed into a visible image (toner image) with toner, and the visible image is transferred onto a recording medium. After transfer of the toner image, residual toner remains on the image carrier. When the image carrier having such residual toner is subject to charging, the image carrier cannot be uniformly charged due to the residual toner. Therefore, generally, after transfer of a toner image, toner and others remaining on the image carrier are removed by a cleaning unit, such as a cleaning blade, so that the surface of the image carrier is satisfactorily cleaned and prepared for the following charging.
The surface of the image carrier is exposed to various physical stresses or electrical stresses at the process of charging, development, transfer, cleaning, and others, and the surface state changes with the passage of use time. Among the stresses, stresses due to friction at the cleaning wear the image carrier to cause a fretting damage. For solving this problem, there have been proposed technologies to reduce the frictional force between the image carrier and the cleaning blade with respect to the lubricant, the method for feeding a lubricating component, and the method for forming a lubricant film.
For example, Japanese Patent Application Publication No. S51-22380 discloses a technology in which a solid lubricant mainly containing zinc stearate is fed to the surface of a photoconductor to form a lubricant film for extending the life of a photoconductor or cleaning blade.
Japanese Patent Application Laid-open No. 2005-274737 describes that, when a lubricant to be fed includes a higher alcohol having 20 to 70 carbon atoms, the higher alcohol remains in the form of indefinite particles at the tip of a blade nip portion, and has appropriate wettability to the surface of the image carrier to keep the lubricating properties.
Japanese Patent Application Laid-open No. 2002-97483 has proposed the use of powder of a specific alkylenebisalkyl acid amide compound as a lubricating component. With this, the powder is present between a cleaning blade and the surface of an image carrier, and an lubricating effect can be maintained for a long term.
As mentioned above, the image carrier is exposed not only to stresses applied at the cleaning but also to electrical stresses, and particularly the electrical stresses applied at the charging operation considerably change the surface of the image carrier. The electrical stresses are remarkable in a contact charging system or near charging system such that a discharge phenomenon occurs near the surface of the image carrier. In these charging systems, a large amount of active species or reaction products are caused on the surface of the image carrier, and active species or reaction products generated in air in the discharge region markedly adsorb on the surface of the image carrier.
Therefore, the lubricant disclosed in Japanese Patent Application Publication No. S51-22380 using zinc stearate relatively uniformly covers the surface of the image carrier to achieve excellent lubricating properties. However, when the resultant lubricant layer is repeatedly subject to the charging operation, stearic acid is decomposed and eventually the zinc oxide formed remains on the surface of the image carrier or the surface of the charging member. The remaining zinc oxide has moisture absorption, and moisture in air adsorbs on the zinc oxide to lower the resistance, and therefore electrostatic charges cannot keep on the image carrier in a high-humidity environment and the resultant electrostatic latent image is not clear, causing an image defect, i.e., so-called image blur.
The lubricant disclosed in Japanese Patent Application Laid-open No. 2005-274737 having a higher alcohol has wettability to the surface of the image carrier and is expected to serve as a lubricant. However, it is likely that the higher alcohol molecules adsorbing on the image carrier have a large adsorption occupying area per one molecule, and hence the density of the adsorbing molecules per unit area of the image carrier (weight of the adsorbing molecules per unit area) is small. Therefore, the electrical stresses easily penetrate the protective layer, making it difficult to protect the image carrier satisfactorily.
In the lubricant of Japanese Patent Application Laid-open No. 2002-97483 containing a nitrogen atom in its molecule, when the electrical stresses are applied to the lubricant, the lubricant itself forms a decomposition product formed of an ionically dissociating compound, such as a nitrogen oxide or an ammonium-containing compound, and the decomposition product is drawn in the lubricant layer, so that the lubricant layer is reduced in resistance at a high humidity, causing image blur.
An image forming apparatus or components used in the image forming apparatus having a long life leads to the reduction of the running cost and the reduction of waste resulting in protection of the earth environment, and therefore, attracts attentions from the market. For example, Japanese Patent Application Laid-open No. 2004-302451 discloses an image carrier having a long life with a specific layer of a crosslinked structure on the surface thereof to improve its mechanical durability.
As mentioned above, for removing the low-resistance substance drawn in the lubricant layer of the image carrier, the whole lubricant layer needs to be removed by, for example, a cleaning mechanism. However, large force is needed in the removal due to the lubricity of the lubricant layer, and further a great amount of mechanical stresses are applied to the image carrier during the removal of the lubricant layer. Therefore, even when a specific surface layer having a crosslinked structure is formed on the surface of the image carrier as disclosed in Japanese Patent Application Laid-open No. 2004-302451, the resultant image carrier does not have a life longer than ever.
Recently, toner produced by polymerization (polymerized toner) for improving the image quality or reducing the production energy is on the market. The polymerized toner has excellent features such that it has less sharp edges and has a small and uniform average particle diameter, as compared to toner produced by grinding (grinded toner). However, in a system in which the edge portion of a cleaning member, such as a cleaning blade made of a rubber, is pressed against the surface of the image carrier to clean the surface of the image carrier, the polymerized toner is difficult to remove by the edge portion due to the form and particle diameter of the toner, leading to a problem in that the remaining toner component is likely to cause a cleaning failure.
For example, Japanese Patent Application Laid-open No. 2000-330441 discloses an image forming apparatus including a conventional cleaning device that can remove such a toner-cleaning failure. In the image forming apparatus, pressing force is controlled using, for example, a volume average particle diameter D and an average circularity S of toner so that the pressing force meets predetermined conditions. When a pressing force f of a counter-type cleaning blade is increased, the cleaning blade disadvantageously squeals or turns over, and therefore, the upper limit as an empirical value needs to be set.
Japanese Patent Application Laid-open No. 2005-99125 discloses a conventional cleaning device that cleans toner having a smaller average particle diameter in a shape close to sphere. In the conventional cleaning device, the coefficient of friction between toner and an image carrier, the coefficient of friction between the toner and a blade, the adhesion between the toner and the image carrier, a force applied to the toner by the blade, and an angle between the blade and the image carrier (cleaning angle) are individually set.
With the conventional cleaning devices, cleaning properties for polymerized toner which is a representative example of a spherical toner are improved with less stress on the image carrier from the cleaning mechanism. However, the service life of image carrier is not taken into account by reducing electrical stresses on the image carrier, and there is a room for improvement.